Electrical cables which are used for supplying electrical energy to submersible oil well pumps must be able to survive and perform satisfactorily under extremely adverse conditions of heat and mechanical stress. Ambient temperatures in oil wells are often high and the I.sup.2 R losses in the cable itself add to the ambient heat. The service life of a cable is known to be inversely related to the temperature at which it operates. Thus, it is important to be able to remove heat from the cable while it is in these extreme operating environments.
Such cables are subjected to mechanical stresses in several ways. It is common practice to fixedly attach the cables to the housing of the electrical submersible pump or oil well tubing by means of bands or straps which may crush the cables and thereby seriously degrade the effectiveness of the insulation and strength of the cable. The cables may also be subject to impact damage during installation and high compression loads during and after installation, particularly when the cable is inserted into wells that do not have perfectly straight bores.
It is therefore conventional to provide such cables with external metal armor anad to enclose the individual conductors within layers of materials chosen to enhance strength characteristics of the cable, but such measures are sometimes not adequate to provide the necessary protection.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. Typically, the insulation surrounding the conductors in a cable contains micropores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the wall, there is not sufficient time for the intrapore pressure to bleed off. As a result of this decompression, the insulation tends to expand outwardly like a balloon and can rupture, rendering the cable weakened or useless thereafter.
In U.S. Pat. No. 4,409,431 in which the assignee is the same as the assignee of the instant invention, there is described a cable structure which is particularly suitable for use in such extemely adverse environments. The structure protects the cable against inwardlydirected compressive forces and provides for the dissipation of heat from the cable which is an important feature in high temperature operating environments, for reasons discussed therein, as well as resistance to decompression expansion of the insulation.
As described in U.S. Pat. No. 4,409,431, the cable protective structure includes one or more elongated force-resisting members which extend parallel and adjacent an insulated conductor comprising the cable. The members are rigid in cross-section to resist compressive forces which would otherwise be borne by the cable conductors. For applications requiring the cable to undergo long-radius bends in service, the elongated support may be formed with a row of spaced-apart slots which extend perpendicularly from one edge of the member into its body to reduce the cross-sectional rigidity of the member in the slotted areas so as to provide flexibility in the support to large-radius bending about its longitudinal axis.
As described in my copending patent application Ser. No. 390,308 filed June 21, 1982 and assigned to the same assignee as the present invention, for certain service applications, it may be preferred that the electrical insulating sheath on the cable conductor not be in direct contact with the slot openings. This is because the slot openings in the support member may allow highly corrosive materials to gain access to the jacket composition by flowing inwardly through the slots. In addition, the corners formed by the slots may cut into or abrade the underlying cable jacket upon repeated bending of the cable.
The cable protective structure of said copending application Ser. No. 390,308 is made of a composite structure which utilizes an elongated force-resisting member of good thermal conductivity positioned adjacent the insulating conductor sheath. This member comprises a channel member of U-cross-sectional shape. A smooth, bendable liner may be mounted within the channel facing the insulation of the adjacent conductor to bridge the slots in the member and thereby protect the underlying insulation from abrasion by the slot edges during bending of the channel member.
The exterior jacket or armor, the liners and the channel members all serve to protect the conductor insulation, and hence the cable, from damage caused by compression forces, impacts and decompression expansion.
Supplementary resistance to compressive forces may be obtained with a cable constructed in accordance with my copending applications Ser. Nos. 429,530 and 429,781, filed on Sept. 30, 1982 and assigned to the same assignee as the instant invention.
For certain service applications and particularly suitable oil well applications, the cable must be able to be axially inserted and withdrawn through an open space formed between the interior circular wall of the well casing and the exterior surface of the oil well tubing, electric submersible pump housing or other structure to which the cable is affixed. Typically, the cable is mounted on the exterior surface of a centrifugal pump and hence, extends outwardly of the pump housing thereby posing a potential obstruction to a proper fit in the oil well casing. Furthermore, it follows that the thicker the cable in cross-section, the smaller the cross-sectional dimension that the pump must have for both to fit into an oil well casing of a given cross-sectional size. Electrically-powered centrifugal pumps, however, are typically much more efficient in larger diameters, and thus, it is preferred that the cross-sectional thickness of the associated cable be made as small as possible so that the user can employ the most efficient pump. Since these structures are typically cylindrical, the open space between them is essentially annular in cross-section, being defined by two essentially circular surfaces of different radius.
As mentioned above, for these applications, the cable is subject to very high temperatures and pressures, severe compressive forces in the well and impacts during installation from, for example, hammers or other tools. Hence, it is desirable to use the cables disclosed in my aforementioned patent applications and yet, it is also desirable to minimize the effective thickness of the cable and thus, the possibility that the cable will jam or lodge against the well casing during the insertion or withdrawal of the equipment to which the cable is affixed.
According to this invention, the armored cable is formed with an arcuate cross-sectional shape which conforms to the curvature of the surface on which it is mounted. This reduces the effective thickness of the cable by conforming to and taking maximum advantage of the annular space available between the wall of the well and the cylindrical structure therein.